The present invention relates to a process for fabricating a low operating current semiconductor laser device and, more particularly, to a process for fabricating such a laser device which uses an indirect transition material absorbing a small quantity of light for the current-blocking layer of the semiconductor laser device to reduce the operating current thereof while allowing precise etching of the current-blocking layer with the clad layer thereof prevented from oxidizing.
Nowadays, semiconductor laser devices are being widely used as light source of optical information equipment. Among these, semiconductor laser devices for use in portable-type CD players, MD players or the like are desired to operate at a low current in view of the life of a battery used therein.
There has been reported at, for example, an autumn meeting of Japanese Applied Physics Society, 17a-v-1 in 1992, a semiconductor laser device wherein light absorption is suppressed at the current-blocking layer as one approach to reduce the operating current. A semiconductor laser device 21 shown in FIG. 3 is one example of this type.
The semiconductor laser device 21 includes, stacked sequentially on a semiconductor substrate 2 such as made of n-GaAs, a lower clad layer 3 such as made of n-Al.sub.x Ga.sub.1-x As, an active layer. 4 such as made of Al.sub.y Ga.sub.1-y As (x&gt;y), a first upper clad layer 5 such as made of p-Al.sub.x Ga.sub.1-x As, Al.sub.x Ga.sub.1-x As, a second upper clad layer 8 made of p-Al.sub.x Ga.sub.1-x As, and a contact layer 9 such as made of p-GaAs. In addition, the lower surface of the semiconductor substrate 2 and the upper surface of the contact layer 9 are formed with a negative electrode 10a and a positive electrode 10b, respectively. The proportion of Al in the active layer 4 is set lower than that in the lower and upper clad layers so that the active layer would be made larger in refractive index and smaller in forbidden band width than the lower and upper clad layers. Further, an optical waveguide is provided so as not to let light produced at the active layer escape toward the clad layer, while in addition the clad layer is adapted not to absorb the light. As described in, for example, Makoto Konagai, "An introduction to superlattice", Baifukan Co., p. 28, 1978, it is known that the refractive index of Al.sub.q Ga.sub.1-q As increases and the forbidden band width thereof decreases with decreasing ratio q. Accordingly, appropriate selection of ratio q makes it possible to obtain desired refractive index and forbidden band width.
As shown in FIG. 3, a current-blocking layer 7, such as made of n-Al.sub.z Ga.sub.1-z As (z&gt;x), formed on the first upper clad layer 5 is partially removed in a striped fashion to form a stripe cavity 11 having a width W.sub.1 and a length L. Since the current-blocking layer 7 is formed of an indirect transition material, light absorption in the semiconductor laser device 21 is suppressed. Further, an optical waveguide is defined along the stripe cavity since the refractive index of the current-blocking layer is lower than that of the clad layer adjacent the stripe cavity.
One example of the process for fabricating this conventional semiconductor laser device 21 is to be described with reference to FIGS. 4(a) to 4(d).
As shown in FIG. 4(a), using an MBE system a first growth layer 20 is initially formed on a semiconductor substrate 2 of n-GaAs by sequentially stacking thereon a lower clad layer 3 of n-Al.sub.x Ga.sub.1-x As (x=0.60), active layer 4 of Al.sub.y Ga.sub.1-y As (y=0.15), first upper clad layer 5 of p-Al.sub.x Ga.sub.1-x As, current-blocking layer 7 of n-Al.sub.z Ga.sub.1-z As (z=0.75) and surface-protective layer 12 of non-doped GaAs.
In turn, as shown in FIG. 4(b), a stripe cavity 11 is defined by covering the surface-protective layer 12 with a photoresist film 14 except for the region dedicated to the stripe cavity 11 and etching that region down to the lower surface of the current-blocking layer 7 through the surface-protective layer 12 and current-blocking layer 7 with a sulfuric acid-based etchant using the photoresist film 14 as a mask.
The resulting structure is subsequently placed in the MBE system again, then heated at about 740.degree. C. while irradiated with arsenic molecular beam as shown in FIG. 4(c).
In general, as the temperature rises, the evaporation rate of GaAs increases. However, the evaporation rate of AlGaAs varies little. Accordingly, when the temperature rises, the surface-protective layer 12 of GaAs is evaporated while the first upper clad layer 5 and current-blocking layer 7 are evaporated little. Thus, the surface-protective layer 12 can be removed without affecting the first upper clad layer 5 and current-blocking layer 7.
In turn, the temperature of the semiconductor substrate 2 being kept at about 600.degree. C., a second upper clad layer 8 of p-Al.sub.x Ga.sub.1-x As and a contact layer 9 of p-GaAs are sequentially deposited on the substrate thus processed by MBE process as shown in FIG. 4(d).
Finally, the lower surface of the semiconductor substrate 2 is lapped, followed by forming a positive electrode 10b connecting to the contact layer 9 and a negative electrode 10a connecting to the semiconductor substrate 2. Thus, the semiconductor laser device 21 shown in FIG. 3 is completed.
With such a conventional process, however, the formation of the stripe cavity is performed by chemical etching and, hence, an etching depth cannot accurately be controlled. This causes a problem that even the active layer is undesirably etched by too much etching or that the current-blocking layer is left too much by insufficient etching to inhibit current from flowing. In addition, the current-blocking layer and clad layer are of mixed crystal having a relatively high Al ratio and hence are likely to be oxidized during the chemical etching. This can be a factor of degraded device reliability.
It is, therefore, an object of the present invention to overcome the foregoing probolems and to provide a process for fabricating a semiconductor laser device operable at a low current with a high yield and a good mass productivity.